US2496484A - Magnetostrictive phonograph pickup - Google Patents

Description

Feb. 7, i950 F. MASSA MAGNETosTRIcTIvE PHONOGRAPH PICKUP Filed April 17, 1946 Patented Feb. 7, 1950 oFFic'E i MAGN ETOSTRICTIV E PHONOGRAP PICKUP Frank Massa, Cleveland Heights, Ohio Application April 17, 1946, serial No. 662,791

My invention is concerned with electro-mechanical transducers of the type in which vibrating mechanical motions may be converted to alternating electric currents, and, conversely, alternating electric currents may be converted to mechanical motions. This invention is primarily concerned with a phonograph pickup of the magnetostriction type and is a continuation in part of my patent application for Transducer means, Serial No. 588,691, led April 16, 1945, now U. S. Patent No. 2,475,148.

As explained in the above patent application, the magnetostriction effect is the well-known magnetic phenomenon in which certain substances show a change in their magnetic properties when subjected to a mechanical strain. A reciprocal magnetostriction effect in these same substances causes their length to change when they are subjected to a change in magnetic field.

The purpose of this invention is to provide a means for eiciently employing the magnetostrictive properties of materials in the design of electro-mechanical transducers and, in particular, to produce a phonograph pickup which has performance characteristics superior to those shown by conventional electro-magnetic pickups and also in which the manufacturing cost is considerably less than the cost oi such conventional pickups.

The conventional magnetic pickup generally employs a high permeability magnetic armature which is usually pivoted between one pair of pole tips and free to move between another pair of pole tips. The region within which the armature is free to move between a pair of pole pieces is usually of the order of a few thousandths of an inch, so that extreme care is necessary in the construction of the device as well as in the balancing of the armature so that the neutral position will remain exactly in the magnetic center of the pole pieces. As the magnetic armature is vibrated in its assembly, alternating ux is caused to ow through the armature which, in turn, induces a voltage within a small coil of wire which surrounds the vibrating armature which, in turn, furnishes the audio signal for operating the amplifier and associated phonograph equipment. The very nature of the design of the magnetic pickup requires precision production methods which result in relatively high costs. Due to the fact that a relatively high stiiness is necessary to keep the armature balanced and to prevent it from being attracted to one pole piece or the other, it is usually necessary to provide 14 Claims. (Cl. NS2-,100.411)

needle :forces of several ounces to permit satis-I factory tracking of 'the needlein the record The high cost of magnetic pickups, coupled with their higher tracking pressures, vcaused them to becomesuperseded commercially by a Rochelle' salt type'of crystal pickup which has been produced at appreciably lower vcost and permits greater Acompliance at the needle point than has been possible with the conventional magnetic pickup. In' View 'of these advantages of the crystal pickup, it has found widespread application in spite of the fact that the structure is inherentlyV fragile and 'suifers from tern-` p'erature and humidity conditions that are normally lencountered over portions of the United States; Another disadvantage of the crystal pickup, which is tolerated because of its lower cost, is its' internally high electrical impedance requiring the use of a high impedance volume control that is generally less desirable than lower impedance control devices.

VMy phonograph pickup overcomes al1 the lobjections of the magnetic pickup and is also superior inl every respect to the crystal pickup which has, up to now, widely replaced the magnetic units. My new pickup has a high enough needle point compliance so that satisfactory tracking results with a needle force of about one ounce. gap of the order of 1A may bel used which eliminates all'magnetic balancing problems as well as all expensive close tolerances in the assembly, and the efliciency is such that an output of the order of 1/3 volt is produced at an impedance level of about` 10,0 00y ohms. Forconstant velocity of the needle" tip, my new pickup gives essentially constant output over the frequency range cycles to '7000 cycles and the high-frequency range may be easily extended to higher frequencies for the few applications that may require it provided some sacrifice in output wil be permitted. f f The vibrating structure of my new improved pickup is a bi-metallic magnetostriction reed similar in principle to the transducer elements described in my aforementioned patent application.` Theiphysical constants of the vreed structure that had to be developed to produce the desirable pickupv characteristic will bedescribed later in this specification. The mechan- Its construction is such that a single air ical simplicity of the unit will be evident from a detailed consideration of the specifications which follow.

In the conventional pickups heretofore in general use, the needles have either been of the relatively heavy replaceable type held in position by a set screw or of the long-life, light-weight type wherein the needles are not easily interchangeable. One ofthe features of my new pickup is a light-weight, long-life needle tip that is easily replaceable by the user without resorting to the use of special tools or set screws.

An object of my invention is to producea.low cost, magnetostriction transducer that may be used for the conversion of mechanical motion into electrical energy, or for the conversion of electrical energy to mechanical motion over the normal audio-frequency range.

Another object of my invention is to produce a phonograph pickup which is much simpler to manufacture than the conventional magnetic pickups.

Still another object of my invention is to produce a phonograph pickup which will play phonograph records with so-called light needle pressures which are in the neighborhood of one ounce to one-and-a-quarter ounces.

Another object of my invention is to produce a pickup which is extremely rugged and not subject to variations dueto climatic changes.

A further object of my invention is to provide an improved pickup design employing a long-life. light-weight needle tip which is easily replaceable by the consumer without the necessity of special tools.

Another object of my invention is to produce a phonograph pickup of improved performance characteristic such that high-fidelity reproduction maybe obtained from high-quality disc recordings.

Still another object .of -my invention is to produce a phonograph pickup in which Iundesirable stray magnetic fields are neutralized to reduce hum therein.

The novel features that I consider characteristic of my invention are set forthwith rparticularity in theappended claims. The invention, itself, however, both as 4to its organization and method of operation as well as additional objects and advantages thereof willlbest be understood from the following description .of v.several embodiments thereof when read in connection, with the accompanying drawings, in .which- Fig. 1 is a plan view of a bi-metallic. magnetostrictive reed that I employ in mynewimproved electro-mechanical transducer assembly.

Fig. 2 is an edgewise view of the reed of Fig. 1.

Fig. 3 shows the plan view of another type of Toi-metallic, magnetostrictive reed in which one end is tapered to a smaller width.

Fig. 4 is an edgewise view of thereed of Fig. 3.

Fig. 5 shows a plan view of another bi-metallic, magnetostrictive reed in which one end .is tapered to a smaller width in a somewhat different fashion than the taper shown in Fig. 3.

, Fig. 6 is an edgewise view of the reed shown in Fig. 5.

Fig. 7 shows the mode of vibration of a thin, rectangular reed clamped atone-endand driven at its fundamental resonant frequency at its vfree end.

Fig. 8 shows the modeof vibrationof the:same reed indicated in Fig. 7 when -the free end is 4 driven at the frequency corresponding to its rst overtone.

Fig. 9 shows schematically how mechanical damping elements are provided to prevent the mode of vibration indicated in Fig. 8 from being established.

Fig. l0 shows the free shape of one of the mechanical damping pads employed in Fig. 9.

Fig. 11 is a front-end View of a new improved phonograph pickup employing the-teachings of my invention.

Fig. 12 is a side view of the same pickup indicated in Fig. 11.

Fig. 13 is a rear-end view of the pickup indicated in Fig. 12.

Fig. 14 is a cross-sectional view on a slightly enlarged scale of the pickup of Fig. l2 taken along the line Irl-44.

Fig. 15 is an enlarged View of the end of the needle holder shown as part of the pickup of Figs. 11 and l2.

Fig. 16 is an enlarged View of the needle tip assembly as shown in Fig. 11 and Fig. 12.

Fig. 17 is a cross-sectional View through an improved phonograph' pickup employing two of the basic structures indicated in Fig. 14.

Fig. 18 is a front-end view of the pickup shown in Fig. 17. A

Fig. 19 is a schematic wiringdiagram of the pickup of Fig. 1'7 and showing the coilarrangement and the mechanical arrangement of the vibrating elements to achieve the desired objective of eliminating hum pickup in the device.

Fig. 20 is a schematic View of a phonograph pickup embodying my invention.

Referring more particularly'to Figs. l and 2,

the reference character l indicates a bi-metallic assembly consisting of a magnetostrictivelyactive element 2 bonded securely 'to another element 3. The portion 2 Vof the bi-metaliic assembly may be ethernickel or nickel-iron alloy'having approximately 35 per cent to 60 per cent nickel, or cobalt-iron alloy having approximately 20 per cent to '70 per cent cobalt, which is described in` my co-pendingapplication Serial No. 588,691, led April 16, 1945. The element3 may be a magnetostrictively inert material, but is preferably a magnetostrictive lmaterial whose magnetostrictive eflect'is the negative of the effect existing in the magnetostrictive material 2. A suitable combination, for-example, would beto make the element 2 of 'nickel and-the element 3 of nickel-iron alloy having approximately 35 per cent to 60 per cent nickel in its composition. Anothersatisfactory arrangementwould be to have the element 2 nickel and the element3 of cobaltiron alloy having from 20 percentto '70 per cent cobalt in its composition. Theelements -2- and 3 are bonded together by any suitable-'method `to form the composite magnetostrictivefelement ll. Infexperimental tests which I have-conducted, I found that the output of a phonograph pickup would increase several fold if the magnetostriction element was completelyannealed to the deadsoft state after fabrication.

Figs. 3 and 4 show abi-metallic reed llA which is builtup from an assembly of portions 2 and 3 the same as described in connection with Figs. 1 and 2. The element 4 is tapered, as shown, whereas the element l is or uniform width .throughout its length.

Figs. 5 and Gshow a bi-metallic element 5 which is also similarly constructedfrom components 2 and 3 in the same manner as describedin Figs..l and 2. The element 5 diifersfrom theelement 4 of Fig. 3 in that the taper in element 4 results from removing material only from one side of the parallel edges as shown in Fig. 1, whereas the element 5 is tapered by removing material from each side of the parallel edges of the element I of Fig. 1.

The reason for having a taper as shown in Figs. 3 and 5 is that the effective mass of the vibrating system is reduced over the system of Fig. 1 at the higher frequencies. Another advantage of the taper results from the fact that the bending stress which is produced along the length of the reed when the free end is deflected is more uniform for the tapered system, thereby making each part of the tapered reed more uniformly active in the generation of voltage than would be the case in the uniform reed of Fig. 1.

Both elements of the bi-metallic assembly in Figs. 2, 4 and 6 are shown of equal thicknesses which would be desirable if each component material has the same modulus of elasticity. In such a case, by bending the free end of the reed, the boundary between the two component elements will remain unstressed and one element will be completely in tension while the other will be completely in compression. If the component elements are not of equal moduli of elasticity, the stiffer material should be thinner than the shofter material, such as indicated in Fig. 11 of my aforementioned co-pending application. The ratio of the thickness should be adjusted so that the boundary between the two elements remains unstressed during bending.

Fig. 7 illustrates diagrammatically they mode of vibration of a fiat, rectangular reed I which is clamped at one end between the rigid blocks 6 and driven at the opposite free end at any frequency up to its fundamental resonant frequency. Ihe fundamental frequency of a thin, rectangular reed which is clamped at one end, as indicated in Fig. '7, is given by the well-known expression i 12p cycles per second where t=thickness of reed in cms.,

Z=free length of reed in cms.,

Q=Youngs modulus of dynes/cm.2,

p=density of material in grams/cc.

Fig. 8 shows the schematic representation of the mode of vibration of the same reed indicated in Fig. '7 at the rst over-tone above the fundamental resonance of the reed. For a thin, rectangular reed, the first over-tone occurs at 6.27 times the fundamental resonance frequency.

In the conventional design of a vibrating system including a clamped reed, it is the general practice to so dimension the reed element that the fundamental resonance occurs at a frequency above the region within which the device is to be used. The reason for this procedure is to avoid deviations from the simple mode of vibration, as indicated in Fig. 7, and thus maintain uniform response over the frequency range of interest. If this procedure were followed in the design of my new pickup which will be described below, the stiffness of the clamped reed would be much too high to permit tracking of the pickup unless very high pressures were employed for keeping the needle in the record groove. This situation would be very undesirable both because of the abnormally high record and needle wear that would result and also because of the very the material ln high noise that would be generated by the needles scraping through the record grooves. In fact, the pressures would be so high that the pickup would be commercially inoperative. In thedesign of my new pickup, I nd that in order to have a high enough compliance at the needle point so that tracking will be possible with a needle force of from one ounce to one-and-aquarter ounces, and if the high-frequency response is to extend to the region 5 kc. to 8 kc., it is necessary to provide a reed thickness in the range 1/2 mm. to 1 mm., and for such a condition, I found that the fundamental natural frequency of the vibrating system lies somewhere in the region within plus or minus one octave of 500 cycles. y

I also found it possible to deviate from the preferred reed construction which has a fundamental resonance in the region within plus or minus one octave of 500 cycles, provided that either the output voltage were reduced and the high-frequency range extended beyond 8 kc., or provided the high-frequency range were limited to a value less than about 3 kc. For these special conditions, the resonance frequency of the vibrating reed could be as high as 2000 cycles for the form'er condition and as low as 200 cycles for the latter condition. When the fundamental resonance of the reed occurs in the lower audio range, it is obvious that the mode of vibration indicated in Fig. 8 will tend to be established somewhere along the higher portion of thel audio reproduction range for which the pickup is intended, and if this higher mode of vibration were permitted to take place, the response of the pickup would not be uniform at the higher frequencies. In order to' prevent the breaking up of the reed into the higher modes of vibration, such as indicated in Fig. 8, I employ a specially designed damping pad I I shown in its normal shape in Fig. l0. The damping pad. II comprises a simple at spring 1 which is coated with a layer of soft, viscous material 8, such as a heavily plasticized vinyl resin or cellulosic resin.

Referring to Fig. 9, the reed I is anchored to a portion of a base I0 by means of the drive screws 9. Along each side of the reed I is a damping pad I I held in position against the reed by the slightly flattened spring 'I which is mounted between the reed I and the base I0. The pads II4 are placed in such a manner that the soft, viscous material layer will be kept pressed against a portion of the reed I. With such an assembly, I nd that when the free end of the reed is driven at the higher audio frequencies, the modes of vibration, such as indicated in Fig. 8, are prevented from being established and, as a result, a pickup incorporating the vibrating system shown schematically in Fig. 9 gives a uniform vrresponse characteristic over the entire audio-frequency range for which the pickup is intended. The damping system shown in Figs. 9 and 10 also successfully prevents the torsional vibrational modes of the reed I that would tend to be excited at the higher frequencies as a result of the simplified needle drive system shown in Fig. 12. The torsional modes would tend to be produced because the needle point driving force is being applied along an axis off the center line of the reed as is evident from the construction in Fig. 12.

An alternative arrangement that I successfully employed to drive the reed consisted in pivoting the needle shoe I I about half-way between the needle tip 20 andthe reed I and attaching .monaca fthe needlershoe to the end pf'themed -holyrat a .point mear the 'center fline of the freed'.l With such :a .pivot arrangement, no torsional :modes were excited in the reed vI which simplifiedthe 'dam-ping problem. ..However, -the damping system fasshown vin "Figs. 9 and 10 Wascapable 'of suppressing the undesired modes :and produced a satisfactory pickup :at ya vlower cost than would be required if the pivot drive Asystem 'were adopted. It is, of course, .obviousthat .the'iuseof a .pivot valong the needle .shoe can be employed in the construction, if desired, .without deviating Ffrom'the teachings of my invention. AI'ffthe *.pivot is placed half-'way between fthe 'needle ='tip and the drive point to the reed I, y.there f'w-illfbe no Le;

change required in the design constants ofthe vibrating system. If 'the .pivot causes fthe .lever ratio .between the needle tip and -the 'reed .to be different from unity, -thenthe .reed stiffness will .have vto be modified so that it isincreasedfor decreased by 'the square of the -lever ratio so that the resulting needle .point compliance will re- :main the Isame as would have beenthe `rcase with the :unity -lever ratio system. The `eie'ctivetransvformation of the reed compliance to the :needle v tip by the ysoluarf-z of thelever ra'tioisfa principle Well-known in the art'of 'electro-acoustic vtransducer design.

r'An alternative method -ro'f damping that. Ilfound satisfactory was produced by substitutingfapiece vthe desired objective ina Icleaner'a'nd .lower 'cost assembly.

Still .another type lof `satisfactory damping that I found consisted-ifoffiproviding :a

sponge-like mass of soft, yiscoussubsta'nce in Ythe Iregion `between the reed II -fandlthe Walls of the structure I0.

The actual design of a complete .phonograph pickup, whichhas a uniform response 'charac- -teristic up to about `6500-cycles vwhen thefmeedle tip is driven with constant Velocity, is sh'own in lthe -four lviews represen-ted by Figs. 1.1, ,12,`13,and `.14. Referring to lthese figures, the .reference #character i2 indicateslamolded, plastic `structure which serves ias a coil-form, the mounting base for the magnetostrictivefreed, theterminalboar'd,

Iand also as a support Vfor the polarizingrnagnet The structure I2 lhas a rectangular lopeningalong its length shown best in the viewof Fig..14,and

,an oi-setportion I3- shown inthe views ofFigs.

.12, .13, and 14. Into the oiT-set portion I--3 ofthe structure .I2 .are assembled .the terminal'pins I4 `after the spool-like portionofthe structure I.2..is wound With insulated Wire 'I'.i. The ends I5"'of the coil of Wire .I5 yare then soldered Ito -the 'tips I of the terminal pins I4. The =oiset.portion.l3 Ahas a plane surface to .which the yreed I-is anchored .by means of the -drivescrews 19. The plane suriaceis so located with reference tothe rectangular opening along the .length of .the structure I2 that when the reed...Ifisattachedto the plane surface, -the reed Willlbealigned along thecenter line of the opening. .Apair -of-damping members I I, similar to 'those-shown .in.Fig..10,

are .inserted within .the .rectangular openingof Fri! structure `I2 toform two identical pickups. vcomponent elements 2 and 3 which lmake up the .reeds Lareso .arranged that the similar elements the structure, shown Ain :14, following the arrangementdcscribed in connection :with ;Fig.r9. A 'ipermanent magnet fI'B -placed .along the outer portion uof `the. icoil, as shown, @and .serves to :send the necessary :polarizing Vfluir A.through the magnetostrictive reed assembly I. -I have found that "when :either .nickel or. 35 per cent to 60 .per cent enic-kel-ironal-l'oy is `usedfas :part of the ,reed structure, the. optimum. flux density necessary to produce 2maximum.:sensitivity and minimum distortion :Wasless `than 300 gauss in .the airfgap., AboveBOO ,graues-'considerable distortion was ob- SerLVable-.in thevelectrical signal generated by the pickup.

f'othefree end fof the `reed I in Fig. .12 .isnttached 'a1-.needle holder =I I which :rnayzbesoldered or welded itoithe ti-p of the reed I preferably before :thou-freed is vassembled to the Astructure 'I2. A :small piece vof vviscous rdani-ping .material im is ,preferably cemented 'ft'o the .surface of the needl-em'oun't IFI., as (shown, vin 4order to .prevent any; undesirable vibrational .modes that might be established in A.the 'needle ilnolder when 'the needle A.tipwis "operated by fa phonograph record. Although not shown in'Fig. 112,the `damping ma .terial :I'Bninay :over-hang the :needle shoe 'I FI vand havepart of 'the `over-hanging portion .secured to;az-fixed"point :such vas the ibase structure :I'2.

.The yfree end of -Lthe lneedle.=mount -If'I .is terminate'd'linto 'a formed 'cylindrical fcollar, sho-Wn fenlarged fin Fig. .15, which iacts 4:as `a :spring lchuck int'o'whfich fa rsleeve `I9 :having a collar 19, including a phonograph needle 20,:as shown finfFig. flpfmaybe insertedsandwheld. 'outer housing f2fl which may :be Yformed `in two piecesfsur- :rounds the pickup assembly :with aulayer 'of :felt .2B A.for .the like :positioned .between vt'he housing .andrfth'efinner assembly, A:and: a'pair of insulating bushings .2 Zare provided to :prevent yshorthfcircuitingzfof the pickup terminals yI4.

f-My 'new;needle :design :permits :the f1ieplacement of long-life needles by an unskilleduuser.` l,of the pickup without resorting to vthe complicated use of a soldering iron-.asfis x.generally employed in present types of permanent-type pickups. rEig. 16 shows an enlarged View of the needle tip which comprises the needle-12D y'firmly assembled to a light-weight sleeve =If9 whichispreferalbly aplastic havinga'highratioof modulus cl 'elasticity -to density, such as polystyrene. The needleZU may be any of the several1 longeli'fe'types ranging 'from 'By-.the use of these 'molded needle as- `the needle .structureto disturb the performance or tracking of the-pickup. It .is also possibleto makethe ,sleeve ilfrom either magnesium or :aluminumiwith some .sacrifice in ,performance due lto the..higher .density of these metals compared -With plastic. v

Figs. 1,7, 1-8, and 19 show a pickup construc- .tion that eliminates .the necessity for shielding the structureagainst stray magnetic fields. Re-

l.ferring to .F-ig. 17, two base structures I2 are mounted on acommon base 25 .by ymeans of the screws 26. A. reed I is assembled to each .base The Ysucnasrlace veach other .when the assemblies Y.are arranged as .shown in Fig. 1.7, i.-e.,.the mag- .netostrictive .components of the bimetallic elev.mentsarearranged..in opposite order. Common I and a common magnet IB furnishes polarizing flux simultaneously through both of the reeds. A needle tip 24 is shown directly attached to the common driving means 23. The schematic wiring diagram for the two coils I is indicated in Fig. 19. For the coils connected, as indicated, any stray magnetic flux passing simultaneously through both of the reeds will induce equal voltages in the coils I5, and since they are connected in series opposing, as indicated in Fig. 19, the voltage across the terminals due to the leakage flux will be zero. When the needle vibrations cause the reeds I to vibrate, the voltage generated in the lefthandcoil will be in opposite phase to the voltage generated in the righthand coil due to the fact that the elements 2 and 3 of the bimetal assemblies are reversed, as shown in Fig. 17.

-Due to this phase reversal, the voltages induced in each of the coils I5 will be additive so that audio voltage will appear across the pickup terminals corresponding to the vibrations of the needle 24.

Fig. schematically illustrates a magnetostrictive phonograph pickup in which two styli and 3l are attached to the free end of the bimetallic element I. In the position shown, the pickup is ready to play hill and dale recordings. The pickup head may be mounted in the pickup arm on a swivel arrangement which is not shown, so that a simple twist will cause a S30-degree rotation of the head, bringing the stylus 3l in a vertical axis and the pickup is then ready for playing lateral-cut recordings. The reference character 32 indicates a rigid anchor for the bi-metallic magnetostrictive device I, and the reference character 33 indicates a permanent magnet which serves to send the required polarizing flux through the bi-metallic strip.

It is to be understood that the particular forms yof apparatus shown and described, and the parl. In combination in a phonograph pickup, a

bi-metallic element one portion of which is magnetostrictive, means for rigidly mounting one end of said element, the other end of said element being free to vibrate in directions transverse to the axis of the element, means for producing a polarizing magnetic eld through said element, and a coil surrounding said element, said element being characterized in that the total combined thickness of the bi-metal lies in the region 1/2 mm. to l mm. and that its fundamental resonant frequency lies in the region within plus or minus one octave of 500 cycles.

2. The invention set forth in claim l further characterized in that said polarizing magnetic field is less than 300 gauss.

3. The invention set forth in claim 1 further characterized in that one portion of said bimetallic element is an iron-nickel alloy.

4. The invention set forth in claim 1 further characterized in that one portion of said bimetallic element is nickel.

5. The invention set forth in claim l further characterized in that one portion of said bimetallic element is composed of a magnetostrictive material in the dead-soft state.

6. The invention set forth in claim 1 further characterized in that damping means is provided in close proximity to the major portion of at least one vibrating surface of said element, said damping means being a layer of viscous material and said close proximity between said viscous layer and said element being maintained by means of a bow-shaped flat spring whose convex surface presses the Viscous layer against the surface of said element.

7. In a phonograph pickup, Va hollow tubular coil form having a reed positioning portion adjacent one end thereof, one face of said reed positioning portion being located near and parallel to the center line of said tubular coil form, a coil wound about said coil form, a bimetal magnetostrictive vibratile reed extending through said coil form substantially on the center line thereof, one end of the reed being attached to said face of said reed positioning portion, the

other end of said reed being free vto vibrate with respect to said coil form in directions transverse to the axis of the reed, said attached reed having a resonant frequency of less than 2000 cycles and more than 200 cycles, there being a gap within said coil form and about said reed, damping means disposed within said gap and engaging said reed and the interior of said coil form, and stylus means connected to the freeend of said reed.

8. The invention set forth in claim 7 further characterized in that said stylus means includes a pivot about which said stylus means is made to rotate as the needle tip isvibrated, thereby causing translatory vibrations to be imparted to said magnetostrictive element.

9. In a phonograph pickup a ,hollow tubular coil form, a coil wound about said coil form, a -permanent magnet disposed' valongside the exterior 0f said coil and'supported by said coil form, a bi-metal magnetostrictive reed, at least one element of which is composed of a dead-soft magnetostrictlve material, extending through said coil form substantially on the center line thereof, one end of said reed being rigidly mounted with respect to said coil, the other end being free to vibrate with respect to said coil in directions transverse to the axis of the reed, stylus means connected to the other end of said reed, said reed being characterized in that the total combined thickness of the bi-rnetal lies in the region 1K2 mm. to 1 mm. and that its fundamental resonant frequency lies in the region within plus or minus one octave of 500 cycles, there being a gap within said coil form on either side of said reed. and damping means disposed within said gap and engaging said reed, said damping means comprising a viscous substance in engagement with the sides of said reed.

l0. In a phonograph pickup a, hollow tubular coil form, a coil wound about said coil form, a bi-metal magnetostrictive reed extending through said coil form substantially on the center line thereof, one end of said reed being rigidly mounted with respect to said coil, the other end being free to vibrate with respect to said coil in directions transverse to the axis of the reed, and stylus means connected to the other end of said reed. said reed being characterized in that the total combined thickness of the bi-metal lies in the region 1/2 mm. to 1 mm. and that its funda mental resonant frequency lies in the region within plus or minus one octave of 500 cycles. there being a gap within said coil form on either side of said reed, and damping means disposed within said gap and engaging said reed and the interior of said coil form. said damping means comprising a viscous substance held in engagefundamental resonant frequency lies stylus; means connected to thev other end of said y reed, said reed being characterized in that its in the region` Within plusser minus one octave of 500 cycles, there being a gap Within said coil form on either side of said reed, and damping means disposed Within said gap and engaging said reed and the interior of said coil form,.said damping means comprising a viscousY substance in engagement with' the sides of said reed.

12. In a phonograph pickup a hollow tubular coil form, a coil Wound about said coil form, a biemetal magnetostrictive reed extending through saidlcoil form, one'end'vof said reed being rigidly mounted With respect to `said coil, the other end being free to vibrate With respect to said` coil inl directions transverse to the axis of thev reed, stylus means connected to the other end of said reed, there being a gapffwithin said coil form on eitherv side of saidA reed,said reedr being charquency is less than 2000 cycles and more than 200 cycles, and damping means disposed within said gap andaengaging said reed, said damping means comprising: a viscous substance in engagement with saidV reed` throughoutthe major portion of the length thereof. y f

13. In combination in a, phonograph pickup, a pair of bimetallic magnetostrictive vibratile reed-like elements, a coil surrounding each of said pair of elements, common means for the simul" taneous driving of both said elements, magnetiz ing means for providing magnetic flux through each of said elements, the magnetostrictive components of said elements being arranged in opposite order, said coils being connected in series v acterized in that its fundamental resonant frey opposition to each other, wherebyl the arrangement of said elements and the connection of said coils is such that the voltages induced in each coil due to the simultaneous vibration of the elements are additive Whereas the voltages induced in each coil due to the presence of a stray alternating magnetic eld are subtractive.

14. In' combination in a phonograph pickup, a pair of laterally-spaced parallel bimetallic magnetostrictive vibratile reed-like elements, a coil surrounding each of said pair of elements, common means for the simultaneous driving of both said elements, magnetizing means for providing magnetic ux through each of said elements, the magnetostrictive components of said elements being arranged in opposite order, said coils being connected in series opposition to each other; whereby the arrangement of said elements and the connection of said coils is such that the voltages induced inv each coil due to the simultaneous vibration of the elements are additive Whereas'the voltages induced in each coildue to the presence of a stray alternating magneto iield are subtractive.

FRANK MASSA.

, REFERENCES CITED 'Ihe following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Magneto-striction, by M. J. Cuttler, Radio News for Nov.` 1928 (pages 450-453)

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